This invention relates to the general field of digital communication systems, and more specifically to modulation and demodulation systems such as modems that employ quadrature amplitude and phase shift keying.
Designers of modern digital communication systems typically employ a modulation technique known as amplitude and phase shift keying, also referred to as quadrature amplitude modulation (QAM). When used in modern modulator/demodulator (modem) devices that employ the V.34 protocol recommended by the International Telecommunication Union for communication between computers, the QAM technique yields improved bit rate for a given signal to noise ratio. The transmission/reception rate capabilities of these modern systems are routinely pushed to their limits when asked to transmit and receive large amounts of data that may include digitized text, graphics, and sound. In addition, the desire to operate at low power levels calls for designers to squeeze as much information as possible into a given signal without increasing its power.
In order to appreciate how this invention improves a digital communication system, a brief review of some basic communication concepts is presented. Turning now to FIG. 1, a prior art technique for preparing information at a transmitter to be sent to a communication channel is shown. First, information such as video or audio is often initially in analog format, such that a conversion into digital form is required in order to be transmitted by a digital communication system. This is accomplished in the A/D conversion block 103 in FIG. 1.
Next, the information will often be encoded into another digital format, as done in waveform coder 105. For example, when audio such as speech is digitized by an A/D conversion block 103, the resulting bit stream typically flows at 64 Kbits/second. This bit rate is due to sampling and quantization requirements for maintaining an acceptable level of fidelity between the original analog signal and the digitized version. Such a bit rate may, however, be too high for error free transmission over a channel such as an analog telephone line. Hence, what is known as a waveform coder will be typically used to encode the 64 Kbit/sec. stream into 8 Kbit/sec by taking advantage of certain special characteristics of speech. In so doing, waveform coder 105 often will create packets of information from the steady stream of digital information it received from the A/D conversion block 103. Thus, the output of waveform coder 105 may be a series of frames where each frame consists of n bits, such that one of 2.sup.n different frames can be transmitted at a time.
The output of waveform coder 105 can typically be characterized as two signals: a primary signal 109 and an auxiliary signal 111. Each one is in the form of a series of frames as described above. A significant portion of the audio data is contained in a given primary frame, while a less significant portion lies in an associated auxiliary frame. Such a separation between primary and auxiliary signals is a result of one of many well-known waveform coding schemes used for encoding audio data. In one case, a primary frame may consist of 4 bits such that 2.sup.4 =16 different primary frames can be generated by waveform coder 105. An associated auxiliary frame would consist of perhaps 2 bits representing one of 4 different auxiliary frames.
Once encoded into primary and auxiliary signals 109 and 111, the binary information is again processed to be superimposed on a carrier waveform for transmission along a communication channel. This process is called modulation. Modulation in the purest sense is defined as the alteration of the carrier in order to cause it to convey information. For example, the characteristics of a simple carrier can be expressed in the form of a sinusoid A sin(.omega.t+.phi.), where A is amplitude, .omega. is radian frequency, and .phi. is phase. Thus, a carrier can be modulated in amplitude, frequency, or phase. Digital modulation, which is used in this invention, refers to the use of a limited set of discrete values of A, .omega., and .phi. consistent with the binary nature of the information to be transmitted.
The term quadrature amplitude modulation (QAM) is used to describe the combining of two amplitude and phase modulated carriers in quadrature. Quadrature refers to a phase difference of 90.degree. between the two carriers. One of these carriers is known as the In Phase carrier and the other as the Quadrature carrier. For example, one can be a digitally modulated sine wave, while the other is a digitally modulated cosine wave of the same frequency.
In FIG. 1, auxiliary frame-to-vector mapper 113 for the auxiliary signal, and primary frame-to-vector mapper 115 for the primary signal are shown following the coder 105 as part of a QAM modulator 110. Operation of these two mappers can be best understood by referring to FIG. 2. FIG. 2 is a signal space diagram containing a constellation depicted by the 16 primary symbol points, used to represent the output of a frame-to-vector mapper. The 2-dimensional space is called the In phase and Quadrature (I-Q) plane 221, where a vector drawn from the origin and ending in a point on the signal space represents the modulation to be performed on the quadrature carriers. Points along the horizontal axis represents all possible modulations to a single cosine carrier, whereas points along the vertical axis represent all possible modulations to a single sine carrier. Points in the plane therefore require a combination of a sine and a cosine carriers.
In the exemplary I-Q plane 221, primary symbol point 223 at the endpoint of primary vector 227 is one of 16 different primary symbol points lying on the constellation in the I-Q plane. These primary symbol points identify the modulation to be applied on a carrier by QAM modulator 110 for each of 16 different primary frames 109 of binary information generated by waveform coder 105 in FIG. 1. In a similar fashion, auxiliary vector 229, associated with primary vector 227, represents the modulation to be applied for an associated auxiliary frame 111 generated by the waveform coder 105.
Returning to FIG. 1, in a typical prior art QAM modulation system, the output of the auxiliary frame-to-vector mapper 113 (represented by auxiliary vector 229 in FIG. 2) may need to be amplitude limited by a clipper 107 to avoid introducing detection errors at the receiver. Any auxiliary vector 229 having an amplitude larger than a, where 2* a is the horizontal and vertical distance between two neighboring primary symbol points in the constellation, is redefined as having amplitude a. This can be seen in FIG. 2 where auxiliary vector 229 is drawn beginning at the end point of the primary vector 227 and lies within a disk of radius a.
After the auxiliary and primary vectors have been computed by mappers 113 and 115, respectively, the two vectors are added to form sum modulation vector 231 in FIG. 2. Due to clipper 107 limiting the amplitude of auxiliary vector 229, the endpoint of sum modulation vector 231 will always lie in a disk of radius a centered at the primary symbol point 223. Lastly, the carrier waveform will be modulated in accordance with the sum modulation vector 231 in carrier modulation block 108 and sent across the communication channel.
The significance of the disk having radius a can be appreciated by considering a communication system that supports only a single channel, i.e., a QAM modulator that accepts only primary frames 109. FIG. 3 illustrates a signal space diagram showing an I-Q plane 321 and the symbols received in such a single channel system before being passed to a conventional QAM demodulator. The actual received symbols over a given period of time appear as a cloud formation 341 around each primary symbol point 323. The cloud 341 appears to have a more or less circular shape because of the random nature of noise that was added to the primary vector 327 after modulation at the transmitter. In a noise free environment, detected vectors would have occurred precisely on the constellation, i.e., the designated 16 primary symbol points in FIG. 3. In actuality, however, a small noise vector 329 is added to the primary symbol vector 327 after modulation at the transmitter and before demodulation at the receiver. This results in the formation of the error clouds are around each primary symbol point at the receiver.
It can be seen that most of the contents of error cloud 341 lies within a disk 343 of radius a with center at the primary symbol point 323. If a received symbol point 325 falls inside the disk area around a primary symbol point 323, then the demodulator can safely assume that the received signal consists of the primary symbol 323 represented by primary vector 327, and an additional noise vector 329.
Analogous to this approach, the prior art transmitter of FIG. 1 adds an auxiliary vector 111 to an associated primary vector 109 in order to use the error cloud region to transmit auxiliary information in a second communication channel. The error cloud region is also referred to as a conventional disk of area .pi.a.sup.2. Although the technique of using a conventional disk to support a second channel of communication may be a convenient solution to the problem of transmitting primary and auxiliary information generated by a waveform coder, the performance of a dual channel communication system can be improved by making a more efficient use of the area in the I-Q plane surrounding each primary symbol point.